1. Field of the Invention
The present invention relates to a method of producing a single-crystal silicon film. More particularly, it relates to a method of producing a single-crystal silicon film which is well-suited especially for forming a semiconductor device having a three-dimensional structure.
2. Description of the Prior Art
As is well known, conventional semiconductor devices have been formed in two dimensions within the surface regions of the major surfaces of semiconductor substrates.
However, when it is intended to extraordinarily increase the density of integration of a semiconductor device, a small number of elements can be formed per unit area and a sufficiently high density of integration is difficult to be attained with the conventional semiconductor device formed by arranging the elements in two dimensions.
When, unlike the two-dimensional arrangement of the semiconductor elements in the substrate surface, insulator films and single-crystal semiconductor films are alternately and successively stacked and formed on a semiconductor substrate surface and semiconductor elements such as transistors are formed within each of the single-crystal semiconductor films, the number of semiconductor elements which can be formed per unit area of a semiconductor chip or wafer increases remarkably, and this is very effective for enhancing the density of integration.
In order to realize a semiconductor device having such three-dimensional structure, a single-crystal semiconductor film having good characteristics needs to be formed on an insulator film. There have been proposed several methods of forming the single-crystal film on the insulator film.
One of them is a method called "bridging epitaxy (seeded lateral epitaxy)" in which, as shown in FIG. 1, an insulator film 3 having an opening 2 is formed in the vicinity of the surface of a semiconductor substrate 1, and a polycrystalline or amorphous silicon film 4 is deposited on the whole surface and is thereafter irradiated with a laser beam or electron beam 5 (Japanese Laid-open Patent Application No. 56-73697, and M. Tamura et al., Jpn. J. Appl. Physics, 19, L23, 1980).
This method can employ a continuous-wave (CW) laser (or electron beam) or pulse laser. Here, the case of using the CW laser (or electron beam) will be explained as an example. As illustrated in FIG. 1, first, the part of the polycrystalline or amorphous film 4 deposited on the opening 2 of the insulator film 3 is irradiated with the laser beam (or electron beam) 5. Then, since the polycrystalline or amorphous silicon film 4 is formed directly on the single-crystal silicon substrate 1 in the part of the opening 2, this part of the polycrystalline or amorphous silicon film 4 turns into the single crystal owing to upward epitaxial growth.
Subsequently, the laser beam (or electron beam) 5 is moved in the direction of an arrow 6 while being scanned substantially perpendicularly to the sheet of drawing. Then, lateral epitaxial growth develops to successively single-crystallize the part of the polycrystalline or amorphous silicon film 4 deposited on the insulator film 3.
With the bridging epitaxy as stated above, it is possible to produce the single-crystal silicon film which is continuous from on the surface of the semiconductor substrate to the insulator film.
With this method, however, only one layer of single crystal Si film can be formed by one operation of the irradiation with the laser beam or electron beam. Accordingly, in order to obtain, e. g., a three-layer structure composed of single-crystal Si film/insulator film/single-crystal Si film/insulator film/Si substrate, it is necessary that the structure shown in FIG. 1 is once irradiated with the laser beam or electron beam so as to form one layer of single-crystal film, whereupon the surface of the single-crystal film is oxidized, and a polycrystalline or amorphous Si film is further deposited thereon and irradiated with the laser beam or electron beam as a second operation. With such method, the operations of irradiation with the laser beam or electron beam need to be repeated in a number equal to the number of the single-crystal Si layers to be grown, and the process is extremely complicated. This forms the first disadvantage. The second disadvantage is the variation of a surface shape developing after the irradiation with the laser beam or electron beam. More specifically, when the laser beam or electron beam is scanningly projected, the Si surface layer is once melted and then solidified due to temperature fall, so that the surface of the polycrystalline or amorphous silicon film becomes uneven. Accordingly, when such uneven surface of the polycrystalline or amorphous silicon film is oxidized and the second polycrystalline or amorphous Si film is deposited thereon and is irradiated with the laser beam or electron beam, the surface becomes more uneven. That is, the uneven parts to appear in the surface become greater with increase in the number of times of the irradiation with the laser beam or electron beam.
Needless to say, such single-crystal silicon film having the great uneven parts in the surface is unsuitable for the formation of a semiconductor device having a high density of integration. There has been eagerly requested a method by which a single-crystal semiconductor film having a flat surface of slight unevenness can be formed on an insulator film.